This invention relates to a thermal air flow meter, and, more particularly, to a thermal air flow meter suitable for measuring the quantity of air taken in by an internal-combustion engine.
Various systems for measuring the quantity of air taken in by an internal-combustion engine have been known in the past, such as movable vane types, types utilizing Karman's vortex sheets, and so forth. Thermal air flow meters disclosed in, for example, U.S. Pat. Nos. 3,747,577 and 4,304,128 have recently gained wide application because they usually have a rapid response and can measure the mass flow rate of the air. A thermal air flow meter of the aforementioned type includes a platinum wire of a diameter of between 70 .mu.m and 100 .mu.m stretched within an intake pipe of an internal combustion engine to act as a flow rate detector. A disadvantage of this construction resides in the fact that the thermal air flow meter does not have sufficient durability when the internal-combustion engine is running badly, and the flow meter undergoes mechanical damage due to backfiring.
In, for example, U.S. Pat. No. 4,264,961, an improved thermal air flow meter solving this problem is proposed wherein a part of the air flowing through the intake pipe is led into a by-pass pipe, and the platinum wire, acting as the flow rate detector, is mounted in this by-pass pipe. Since the by-pass pipe has a maximum diameter of 1 cm, the flow rate detector must also be compact. However, since the flow meter measures flow rate by utilizing the phenomenon that the resistance of platinum wire varies with temperature, a higher sensitivity can be obtained by a higher-resistance platinum wire. Accordingly, the flow rate detector is constructed by winding platinum wire around the outer periphery of a piece of insulating material to make the flow meter compact and increase its resistance. With this construction, however, another problem occurs in that the response is lower than that of the system described above, because of the heat capacity of the bobbin used as the support. This problem is not limited to the type of meter which utilizes a by-pass pipe, but also to any meter which utilizes a compact flow rate detector.
The response problem is more critical when the thermal air flow meter described above is, for example, used for a single-point fuel injection system.
In single-point fuel injection, a single injection valve is provided at a point at which the intake pipes of the engine join, and hence the distances from the fuel injection position to the cylinder inlets is longer than those in multi-point fuel injection, and the time taken for the fuel to arrive at the cylinders is longer. Similarly, since the distance from the fuel injection valve to each cylinder differs from cylinder to cylinder, delicate matching must be carried out. Although attempts have been made to compensate for the difference by use of computer software, these have not been entirely successful. After all, in single-point injection, only one injection valve distributes fuel to each cylinder so that delicate matching must be made whenever the model of the car, and hence the shape of the intake pipes, changes. Particularly during acceleration or during high-speed operation, the detection accuracy must be improved so that the pulsating flow of intake air in the engine can be followed precisely, using a highly accurate flow rate sensor.
This problem of response occurs not only in the control of an internal-combustion engine, such as in the single-point fuel injection system, but also in the measurement of flow rates in general if changes in such flow rates are rapid.
It is therefore an object of the present invention to provide a thermal air flow meter which is equipped with a compact flow rate detector, but which still provides a rapid response.
In a thermal air flow meter equipped with a flow rate detector utilizing the phenomenon that heat is carried away in proportion to the flow rate, the thermal air flow meter in accordance with the present invention is characterized in that the flow rate detector comprises a support, a heat-sensitive resistor formed on the support, and leads attached to both ends of the support. The heat-sensitive resistor is formed on the support in such a manner that the resistance per unit length of the heat-sensitive resistor at either end of the support is greater than the resistance per unit length of the heat-sensitive resistor at its center.
Various experiments and studies have been carried out using conventional flow rate detectors of the wound type, and have clarified the following points.
In a conventional flow rate detector, platinum leads which function both as supports and conductors are attached to both ends of a 2 mm-long bobbin made of an insulating material, and platinum wire is wound at constant pitch onto this bobbin. When a current flows through the platinum wire and the heat thus generated is controlled so that the wire is at a predetermined temperature, the temperature distribution is such that it is highest at the center and drops towards the leads. Accordingly, when the set temperature for the flow rate detector is, for example, 170.degree. C., (this can be effected by making the resistance of the flow rate detector a predetermined value), the maximum temperature at the center is about 250.degree. C.
The reason why the temperature distribution is so large can be attributed to the following. In the initial stages when the current starts to flow through the resistance wire, the quantity of heat generated per unit length is the same; but as the heat is transferred, temperature differences occur between the glass coating, the bobbin, and the leads that are in contact with the winding. These temperature differences change the resistance of each part of the resistance wire. For instance, the resistance rises locally at the center, further increasing the quantity of heat generated. When specific structures are examined, it is first of all obvious that the bobbin center is hollow while the two end portions hold the leads and adhesive for the leads, so that they have different volumes which induce differences in heat capacity. Secondly, heat sinks are generated from both end portions because leads of a precious metal are fitted.
In accordance with the present invention, since the resistance of the bobbin is smaller at the center thereof than that at either end, heat generation is less at the center of the bobbin and is more at either end, so that the present invention can provide a thermal air flow meter in which the temperature distribution along the flow rate detector can be made substantially uniform, and which has a rapid response.
The resistance of the heat-sensitive resistor per unit length at the center of the bobbin can be easily made different from that at either end by winding the resistor onto a support coarsely at the center and densely at either end, if the heat-sensitive resistor is a wire, or by making the trimming pitch dense at either end and coarse at the center if it is produced by trimming after the formation of a film.